1. Field of the Invention
This invention relates to an automatic focusing apparatus of camera and more particularly to an apparatus highly suitable for a camera arranged to perform communication of data for control of varied kinds between a camera unit and a lens unit which is detachably mounted on the camera unit.
2. Description of the Related Art
The advancement of video apparatuses such as the video tape recorder (hereinafter referred to as VTR) achieved during recent years has been conspicuous. As a result, the camera-incorporating type VTR or the like has come to include an automatic focusing apparatus as a standard component.
In the case of the camera-incorporating type VTR, particularly in the case where the camera has a zoom mechanism or the like, the characteristic of the automatic focusing apparatus varies with the focal length of the lens. It is therefore necessary to detect focal length information on the side of the lens and to control the characteristic of the focusing apparatus accordingly. The information on the focal length to be used on the camera side in the camera-incorporating type VTR is generally obtained by dividing the focal length range from a wide-angle side through a telephoto side thereof into a number of areas best suited for the lens. Information on the applicable area is detected by an area encoder and is supplied to focus control means.
The following describes, by way of example, a case where the focal length information obtained in the above-stated manner is used by an automatic focusing apparatus or circuit (hereinafter referred to as TV-AF) which detects a focusing state through a TV signal.
FIG. 1 is a block diagram mainly showing automatic focusing means included in a camera-incorporating type VTR. Referring to FIG. 1, light from an object 1 to be photographed is imaged on the image sensing plane of an image sensor 3 by a lens unit 2. The image sensor 3 then photo-electrically converts the image into a sensed image signal. The sensed image signal is supplied to a camera signal processing circuit 4. The circuit 4 generates a TV signal. A chrominance signal and a luminance signal are combined together by an adder 5 and produced as a TV signal.
Meanwhile, a luminance signal component output from the camera signal processing circuit 4 is supplied to the automatic focus control circuit 6. Then, within the circuit 6, a high-pass filter (HPF) 61 extracts only a high-frequency component, i.e., a high-fineness signal, which tends to be generated in a greater amount when the lens is in focus. The output of the HPF 61 is supplied to an analog-to-digital (A/D) converter 62 to be converted into a digital signal. The digital signal is then supplied as focus determining information to a focus determining circuit 63. Further, to determine whether the lens position deviates from an in-focus position in the defocus direction, that is, to make a discrimination between near-focus and far-focus positions in the event of defocus, an actuator 9 which is a bimorph plate or the like is driven via a driving amplifier 8 by a signal which is generated by a modulation signal generating circuit 7 at a frame frequency (or a frequency which is several times as high as the frame frequency or a frequency obtained by dividing it into several parts). This causes the image sensor 3 to vibrate at a minute amplitude forward and backward from an image plane. The focus determining circuit 63 then makes a discrimination between near-focus and far-focus states according to a focus modulation signal which has been modulated by the vibration. The result of determination made by the circuit 63 is supplied to a driving amplifier 10. The driving amplifier 10 then causes a motor 11 to shift the position of the lens 2 to an in-focus position. The minute amplitude by the lens actuator 9 to be made in obtaining the focus modulation signal for detecting the direction of defocus is arranged to vary according to the focal length of the above-stated lens optical system.
In other words, the sensitivity of the optical system varies with the focal length. The degree of defocus does not change much with the focus signal modulated to a small degree if the sensitivity is low. If the sensitivity is high, the focus signal, i.e., the degree of defocus, varies to a great degree in response to a slight degree of displacement of the optical system. Therefore, in order to accurately detect the degree of defocus by detecting the near- or far-focus state and the degree of defocus according to the focus modulation signal obtained by slightly vibrating the image sensor in the direction of an optical axis, the amplitude of focus modulation must be changed according to the focal length. In other words, if a lens position is on the telephoto side, the sensitivity is high and permits a smaller modulation amplitude than in the case of a lens position on the wide-angle side.
The apparatus thus is provided with an area encoder 12 and an area encoder pattern 13 for the above-stated process to be carried out according to the focal length of a zoom lens. The area encoder 12 detects the area encoder pattern 13 to find in which of the divided areas of the variable focal length range the lens position is currently located. A detection value thus obtained is supplied to the automatic focus control (AF) circuit 6. The amplification degree of the driving amplifier 8 is controlled to change stepwise the vibration amplitude of the image sensor 3 from one value over to another for each divided area according to the value supplied.
The above-stated area dividing method adopted for the area encoder 12 and the area encoder pattern 13 is called a floating method. In accordance with the floating method, the areas for controlling the focus modulation amplitude are set stepwise by dividing a range from the telephoto end position or the wide-angle end position of each individual lens either by a given distance or in a given ratio irrespectively of the focal length of the lens obtained at the telephoto end or the wide-angle end of the lens in use.
The above is one example of the conventional camera-incorporating type VTR. As mentioned above, there arises no problem in particular in controlling the characteristic of the automatic focusing apparatus in accordance with the focal length of the lens.
However, the recent advancement of the image sensing apparatuses including VTRs and others has come to prompt an attempt to arrange the video camera and the camera-incorporating type VTR to permit use of an interchangeable lens.
With an interchangeable lens system applied to the VTR or the like, the relation of area setting to the focal length of one lens differs from that of another. Therefore, it is impossible to arrange the control circuit of the camera to ensure sufficient lens information interchangeability among different lenses. The above-stated attempt, therefore, would present a serious problem in terms of automatic focus adjustment.